SAND DUNES AND SALT MARSHES 



Another strong evidence of subsidence is 

 shown in a study of the sections of the 

 marshes, for, as we have seen, the various 

 species of grasses are limited to certain zones 

 in relation to the tides. As remains of grasses 

 are found in the sod sections several feet 

 below the level at which they can now grow, 

 the inference is plain that portions of the 

 marsh formerly stood higher in relation to the 

 tides. Professor Charles A. Davis has made 

 a careful study of the marshes about Boston 

 by means of borings, and he has found even 

 at a depth of twelve feet below the surface 

 of the marsh, peat that was composed of the 

 grasses now growing on the surface. 



It must be remembered, however, that the 

 height of the tides is dependent very much 

 on the character of the shore line, as Professor 

 D. W. Johnson has shown, and as this changes 

 from time to time independently of changes 

 of land level, regions not previously flooded 

 may suddenly be drowned by high tides. This 

 fictitious appearance of coastal subsidence, 

 however, need not blind our eyes to the many 

 incontestable evidences of true subsidence 

 which cannot be so explained. 



As an example of the possibility of errone- 

 214 



